White House hopeful Bernie Sanders claims a planned Cork-Boston air service will herald tens of thousand of job losses in the US and Europe.

The Democratic Party contender wants to prevent Norwegian Airlines International (NAI) from launching the service to the US, claiming it would mark a "dangerous precedent".

US aviation unions claim Norwegian intends using Ireland as a flag of convenience to employ low-paid crew and undermining working conditions for cabin crew working for other transatlantic airlines.

Mr Sanders, who is trailing Hillary Clinton for the party's presidential nomination, has asked the US Department of Transportation not to grant a permit that would allow Norwegian Airlines International (NAI), a subsidiary of Norwegian Air Shuttle, to fly to the US from Ireland, or from other European cities.

"Granting such a permit would be a direct violation of the strong labour provisions included in the US/EU Open Skies agreement," Mr Sanders claimed. "Moreover, it would set a dangerous precedent that threatens the jobs of hundreds of thousands of flight attendants, mechanics, pilots, and other airline workers in our country and in Europe."

The candidate, known for his 'Feel the Bern' slogan, added: "We must do everything we can to prevent a global race to the bottom in the airline industry. If this permit is approved, it would open the door to the same 'flag of convenience' model that decimated US shipping."

Norwegian Air International already has almost 40 aircraft registered in Ireland and plans to start flying from Cork to Boston this year, and to New York in 2017

NAI already has an Irish air operator's certificate, but has waited two years to secure its permit to fly to the US. The US Department of Transportation has delayed the permit in the face of strong opposition from aviation unions.

The unions claim that NAI will use its Irish unit to circumvent stringent labour laws and use cheap employees, including staff from Asia, to the detriment of US airlines.

NAI has consistently denied the claims. A spokesman for the airline said yesterday: "Some US politicians and unions are continuing to do everything they can to block the competition, preventing passengers' access to affordable airfares, and blocking the creation of new jobs and significant benefits to Ireland and the US."

"The fact remains that NAI is a recognised EU airline, with a Dublin headquarters, more than 35 aircraft registered in Ireland and a series of new routes from Ireland planned," the spokesman added.

"It is also a clear fact that Norwegian always follows labour laws in all the markets we operate, offering competitive wages and conditions. NAI does not have a single Asian-based crewmember or pilot."

The unexpected intervention by the veteran US politician comes just weeks after American transport officials said they intended to grant the necessary permit to NAI. Its parent firm, Norwegian Air Shuttle, already flies from the UK to the US.

NAI also plans to fly from Cork to New York next year. Mr Sanders's opposition comes before a looming May 16 deadline for making final submissions to the US Department of Transportation on NAI's plans to use Ireland as its base to fly to America.

Yesterday, Cork Airport managing director Niall McCarthy said the city has "so much to gain" from the planned transatlantic flights.

"We urgently seek the public's help and support to ensure the permit is approved and that the flights are secured once and for all," he said.

Ryanair has also lent its support to NAI, it has emerged. Michael O'Leary's airline told the US Department of Transportation that NAI's services will result in increased competition and consumer choice.

"NAI will deliver a fresh competitive dynamic on the transatlantic routes by offering choice, service improvements and lower fares to US and European consumers, creating jobs and delivering increased passenger volumes," Ryanair says.